1. Field of the Invention
Embodiments relate to the field of device manufacturing. More particularly, the present disclosure relates to an improved deceleration lens used in ion implanters.
2. Discussion of Related Art
Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of devices that employ silicon workpieces. Any reduced cost in the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells which would enhance the wider availability of this clean energy technology.
Some ion implantation processes decelerate the ions prior to implantation. Deceleration is typically performed by applying different combinations of voltage potentials to electrodes disposed on opposite sides of the ion beam. By manipulating the voltage potentials, ion energy can be reduced to a desired level, causing the ion beam to “decelerate.” This allows the ions to be transported at high speed until just prior to implant. Since implant depth is proportional to ion energy, deceleration is often required when forming devices with shallower junction depths.
Such deceleration, however, may lead to energy contamination, which can occur when high-speed neutral particles are formed. Neutral particles are not affected by a deceleration lens because they lack a charge. The neutral particles may form through interactions between ions or interactions between ions and other particles in the implanter. Some workpieces, such as semiconductor wafers, are sensitive to energy contamination because high-speed neutral particles can implant deep into the crystal lattice of the semiconductor wafer. Other workpieces, such as solar cells, are less sensitive to energy contamination. This is because solar cell junctions are substantially deeper than typical semiconductor logic devices, and as such, energy contamination from the implant doesn't affect the final implant profile, subsequent to annealing. Implantation of solar cells may involve high throughput of workpieces at ion beam energies of approximately 1 keV to 10 keV. A deceleration lens is one component in an implanter that may enable high throughputs at these beam energies. Some deceleration lenses may use segmented lenses with adjustable z-positions, which may require multiple power supplies, complicated moving parts, and may be prone to particle deposition on various components.
Deposition of particles on implanter components such as the deceleration lens, can cause a variety of operational problems. Back-streaming particles are one example of particles that may be deposited on surfaces of the deceleration lens that are exposed to, or are in, the ion beam line-of-sight. Back-streaming particles are generated when the ion beam hits the wafer or areas of the ion implanter that are exposed to the ion beam. The impact sputters ions and neutral particles off the impact surfaces, causing the particles to stream back toward the deceleration lens. In one instance, back-streaming particles can be deposited on surfaces of the lens insulators, where the maximum voltage difference is 35 kV for a 2:1 or 3:1 deceleration ratio (such as going from 30 keV to 10 keV). Back-streaming particles also can be deposited on lens bushing plates where the maximum voltage difference across the insulator is 20 kV for a 2:1 or 3:1 deceleration ratio. Any deposition on an insulating surface can reduce the mean time between failures (MTBF) to below-specification levels. When particles deposit on a deceleration lens, glitching which is the sudden transient in beam current, may occur. In addition, high-voltage breakdown of the deceleration lens can occur. High-voltage breakdown is caused when back-streaming particles are deposited on only one side of the lens insulators (i.e., the side facing the end station), thus reducing the insulators' ability to sustain the voltage difference between the electrode and ground. Each of these conditions can adversely affect precise dose control and dose uniformity of the implanted species on a target substrate. Thus, there remains a need for an improved deceleration lens that alleviates the aforementioned problems. The improved deceleration lens should include features that reduce particle buildup on lens surfaces during ion implant operations.